In the case of non-switchable oil separation devices for separating oil from a gas stream for ventilating a crankcase of a combustion engine, referred to as the “blow-by gas stream”, the flow geometry relevant to oil separation is generally invariable over the applied volume flow of blow-by gas. As a result, the oil separation device acts relatively poorly at low volume flows as regards the achievable separation performance, while, at high volume flows of the blow-by gas, the oil separation device does act more effectively but the pressure loss in the oil separation device rises as the square of the volume flow owing to the fixed geometry of the flow path through which the gas stream flows.
In “switchable” oil separation devices, a larger flow cross section for the flow of the gas stream is made available by means of a, generally spring-loaded, element as the volume flow of blow-by gas increases. As a result, separation efficiency is kept relatively constant over a wider range of blow-by gas volume flow. However, the pressure loss across the entire system increases.
At engine load points at which there is a high vacuum in the intake tract, particularly in the case of turbocharged engines, a pressure control valve situated downstream of the oil separation device is used to limit the minimum vacuum in the crankcase of the combustion engine. For example, the vacuum prevailing ahead of the turbocharger of a combustion engine can be up to 60 mbar and that in the intake manifold can be up to 300 mbar. By means of the pressure control valve, the vacuum can be limited to between 30 mbar and 50 mbar, for example. This pressure control is accomplished by increasing the flow resistance across the pressure control valve, resulting in a further reduction in the flow cross section, although this is not accompanied by an improvement in separation performance. As a result, the available flow energy is not used for separation since the gas stream flowing through the pressure control valve has generally already been purged.
DE 10 2004 006 082 A1, for example, shows a cyclone-type oil separation device, in which a plurality of cyclones are arranged in parallel. Depending on the pressure difference, caused by a first cyclone, between the oil-laden gas stream supplied and the purged gas stream, flap valves open the flow path through additional, subsequent cyclones. However, the energy potential which could be obtained for further increasing separation performance remains unused here.
DE 102 05 981 A1 shows another oil separation device, which is constructed with switchable cyclones, and a slide, by means of which openings of the cyclones can be opened or closed, is provided. For this purpose, various apertures are provided in the slide, each being of a different size. If the slide is moved further in an opening direction by a larger quantity of gas volume flow, the inflow openings of the individual cyclones open in such a way that the flow cross sections into the cyclones increase. If the volume flow of the oil-laden gas falls again, the individual flow cross sections of the cyclones close again, but, even with this system, there is no success in enabling the flow energy used for throttling to be used to improve separation performance.